This invention relates to an apparatus for heating an object. More particularly, this invention relates to an apparatus that heats an object both inductively and resistively. The invention is particularly useful for heating large tubular objects, such as barrels and nozzles of machinery, such as extruders and injection molding machines, for processing molten material, such as plastic and metal.
Referring to FIG. 1, a typical resistive heater circuit 10 in accordance with the prior art is shown. A power supply 12 may provide a DC or AC voltage, typically line frequency to a heater coil 14 which is wrapped around in close proximity to a heated article 20. Typically, the heater coil 14 is made up of an electrically resistive element with an insulation layer 18 applied to prevent it from shorting out. It is also common to have the entire heater coil encased in a cover 16 to form a modular heating subassembly. The prior art is replete with examples of ways to apply heat to material and raise the temperature of the heated article 20 to a predetermined level. Most of these examples center around the use of resistive or ohmic heat generators that are in mechanical and thermal communication with the article to be heated.
Resistive heaters are the predominate method used today. Resistive heat is generated by the ohmic or resistive losses that occur when current flows through a wire. The heat generated in the coil of the resistive type heater must then be transmitted to the workpiece by conduction or radiation. The use and construction of resistive heaters is well known and in most cases is easier and cheaper to use than inductive heaters. Most resistive heaters are made from helically wound coils, wrapped onto a form, or formed into sinuous loop elements.
A typical invention using a resistive type heater can be found in U.S. Pat. No. 5,973,296 to Juliano et al. which teaches a thick film heater apparatus that generates heat through ohmic losses in a resistive trace that is printed on the surface of a cylindrical substrate. The heat generated by the ohmic losses is transferred to molten plastic in a nozzle to maintain the plastic in a free flowing state. While resistive type heaters are relatively inexpensive, they have some considerable drawbacks. Close tolerance fits, hot spots, oxidation of the coil and slower heat up times are just a few. For this method of heating, the maximum heating power can not exceed PR(max)=(IR(max))2xRc, where IR(max) is equal to the maximum current the resistive wire can carry and Rc is the resistance of the coil. In addition, minimum time to heat up a particular article is governed by tR(min)=(cMΔT)/PR(max), where c is the specific heat of the article, M is the mass of the article and ΔT is the change in temperature desired. For resistive heating, total energy losses at the heater coil is essentially equal to zero because all of the energy from the power supply that enters the coil is converted to heat energy, therefore PR(losses)=0.
Now referring to FIG. 2, a typical induction heating circuit 30 according to the prior art is shown. A variable frequency AC power supply 32 is connected in parallel to a tuning capacitor 34. Tuning capacitor 34 makes up for the reactive losses in the load and minimizes any such losses. Induction heater coil 36 is typically comprised of a hollow copper tube, having an electrically insulating coating 18 applied to its outer surface and a cooling fluid 39 running inside the tube. The cooling fluid 39 is communicated to a cooling system 38 to remove heat away from the induction heater coil 36. The heater coil 36 is not generally in contact with the article to be heated 20. As the current flows through the coil 36, lines of magnetic flux are created as depicted by arrows 40a and 40b. 
Induction heating is a method of heating electrically conducting materials with alternating current (AC) electric power. Alternating current electric power is applied to an electrical conducting coil, like copper, to create an alternating magnetic field. This alternating magnetic field induces alternating electric voltages and current in a workpiece that is closely coupled to the coil. These alternating currents generate electrical resistance losses and thereby heat the workpiece. Therefore, an important characteristic of induction heating is the ability to deliver heat into electrical conductive materials without direct contact between the heating element and the workpiece.
If an alternating current flows through a coil, a magnetic field is produced that varies with the amount of current. If an electrically conductive load is placed inside the coil, eddy currents will be induced inside the load. The eddy currents will flow in a direction opposite to the current flow in the coil. These induced currents in the load produce a magnetic field in the direction opposite to the field produced by the coil and prevent the field from penetrating to the center of the load. The eddy currents are therefore concentrated at the surface of the load and decrease dramatically towards the center. As shown in FIG. 3A, the induction heater coil 36 is wrapped around a cylindrical heated body 20. The current density Jx is shown by line 41 of the graph. As a result of this phenomenon, almost all the current is generated in the area 22 of the cylindrical heated body 20, and the material 24 contained central to the heated body is not utilized for the generation of heat. This phenomenon is often referred to as “skin effect”.
Within this art, the depth where current density in the load drops to a value of 37% of its maximum is called the penetration depth (δ). As a simplifying assumption, all of the current in the load can be safely assumed to be within the penetration depth. This simplifying assumption is useful in calculating the resistance of the current path in the load. Since the load has inherent resistance to current flow, heat will be generated in the load. The amount of heat generated (Q) is a function of the product of resistance {circle around (R)} and the eddy current (I) squared and time (t), Q=I2Rt.
The depth of penetration is one of the most important factors in the design of an induction heating system. The general formula for depth of penetration δ is given by:δ=√{square root over (ρ/πμμυf)}    where μυ=magnetic permeability of a vacuum    μ=relative magnetic permeability of the load    ρ=resistivity of the load    f=frequency of alternating current
Thus, the depth of penetration is a function of three variables, two of which are related to the load. The variables are the electrical resistivity of the load ρ, the magnetic permeability of the load μ, and the frequency f of the alternating current in the coil. The magnetic permeability of a vacuum is a constant equal to 4Π×10−7 (Wb/A m).
A major reason for calculating the depth of penetration is to determine how much current will flow within the load of a given size. Since the heat generated is related to the square of the eddy current (I2), it is imperative to have as large a current flow in the load as possible.
In the prior art, induction heating coils were almost exclusively made of hollow copper tubes with water cooling running therein. Induction coils, like resistive heaters, exhibit some level of resistive heat generation. This phenomenon is undesirable because as heat builds in the coil, it affects all of the physical properties of the coil and directly impacts heater efficiency. Additionally, as heat rises in the coil, oxidation of the coil material increases and this severely limits the life of the coil. This is why the prior art has employed means to draw heat away from the induction coil by use of a fluid transfer medium. This unused heat, according to the prior art, is wasted heat energy which lowers the overall efficiency of the induction heater. In addition, adding active cooling means like flowing water to the system greatly increases the system's cost and reduces reliability. It is therefore advantageous to find a way to utilize the resistive heat generated in an induction coil which will reduce overall heater complexity and increases the system efficiency.
According to the prior art, various coatings are used to protect the coils from the high temperature of the heated workpiece and to provide electrical insulation. These coatings include cements, fiberglass, and ceramics.
Induction heating power supplies are classified by the frequency of the current supplied to the coil. These systems can be classified as line-frequency systems, motor-alternating systems, solid-state systems and radio-frequency systems. Line-frequency systems operate at 50 or 60 Hz which is available from the power grid. These are the lowest cost systems and are typically used for the heating of large billets because of the large depth of penetration. The lack of frequency conversion is the major economic advantage to these systems. It is therefore advantageous to design an induction heating system that will use line frequencies efficiently, thereby reducing the overall cost of the system.
U.S. Pat. No. 5,799,720 to Ross et al. shows an inductively heated nozzle assembly for the transferring of molten metal. This nozzle is a box-like structure with insulation between the walls of the box and the inductive coil. The molten metal flowing within the box structure is heated indirectly via the inductive coil.
U.S. Pat. No. 4,726,751 to Shibata et al. discloses a hot-runner plastic injection system with tubular nozzles with induction heating windings wrapped around the outside of the nozzle. The windings are attached to a high frequency power source in series with one another. The tubular nozzle itself is heated by the inductive coil which in turn transfers heat to the molten plastic.
U.S. Pat. No. 5,979,506 to Aarseth discloses a method and system for heating oil pipelines that employs the use of heater cables displaced along the periphery of the pipeline. The heater cables exhibit both resistive and inductive heat generation which is transmitted to the wall of the pipeline and thereby to the contents in the pipeline. This axial application of the electrical conductors is being utilized primarily for ohmic heating as a resistor relying on the inherent resistance of the long conductors (>10 km). Aarseth claims that some inductive heating can be achieved with varying frequency of the power supply from 0–500 Hz.
U.S. Pat. No. 5,061,835 to Iguchi discloses an apparatus comprised of a low frequency electromagnetic heater utilizing low voltage electrical transformer with short circuit secondary. Arrangement of the primary coil, magnetic iron core and particular design of the secondary containment with prescribed resistance is the essence of this disclosure. The disclosure describes a low temperature heater where conventional resinous molding compound is placed around primary coil and fills the space between iron core and secondary pipe.
U.S. Pat. No. 4,874,916 to Burke discloses a structure for an induction coil with a multi-layer winding arranged with transformer means and a magnetic core to equalize the current flow in each winding throughout the operational window. A specially constructed coil is made from individual strands and arranged in such a way that each strand occupies all possible radial positions to the same extent.
U.S. Pat. No. 2,163,993 to Dufour discloses an electrical conductor wrapped around an article to be heated, and heating done both resistively and inductively. However, since the coil is on the outside of the article, and there is no part of the article outside of the coil, the magnetic circuit is not closed around the coiled conductor. Also, since there is no part of the article outside of the coiled conductor, some of the resistance heat generated in the conductor is transferred to the surrounding air rather than to the article it surrounds.
British patent 772,424 to Gilbert discloses one or more coils each consisting of a plurality of concentric windings disposed around the cylinder and enclosed in a two piece shell or casing which is also made of magnetizable material and is clamped around the cylinder for an extrusion or injection molding system. The coils inductively heat the cylinder and the shell. There is no disclosure of resistive heating by the coils, nor any detail of the coil construction.
There is a need for an improved heating apparatus that utilizes both the inductive and resistive heat generated from a heating coil located inside the heating apparatus to optimize use of the heat generated therein.